(i) Field of the Invention
This invention relates to a novel polypropylene fiber. More specifically, this invention relates to a polypropylene fiber with high syndiotacticity and a preparation process thereof.
(ii) Description of the Prior Art
Although the existence of syndiotactic polypropylenes has been known from old days, polypropylenes produced by the conventional process, in which propylene is polymerized at low temperatures in the presence of a catalyst comprising a vanadium compound, an ether and an organoalumihum, have been said to have elastomer-like characteristics. However, these polypropylenes are of low syndiotacticity and hence can hardly be regarded as syndiotactic polypropylenes. On the other hand, a polypropylene of good tacticity, say, a syndiotactic pentad fraction of more than 0.7, has been discovered for the first time by J. A. Ewen et al. by the use of a catalyst comprising a transition metal compound having an asymmetric ligand and an aluminoxane (J. Am. Chem. Soc., 1988, 110, 6255-6256).
On the other hand, one of the large uses of isotactic polypropylenes is for fibers, and they have been used as fibers having relatively good properties and strong chemical resistance. However, they are a little inferior in fiber strength and therefore polyolefin fibers improved in this point have been desired.
The present inventors have made intensive investigations into polyolefin fibers which are free from the above problem and hence are excellent in strength, and finally found that polypropylenes of high syndiotacticity are suitable for use as fibers, leading to completion of the present invention.